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The wing panel has not been found, but the NTSB said investigators examined the remaining components and found that metal fatigue had caused two of the three clips securing the panel to fail before the March flight, and the remaining clip failed during the incident flight.

The NTSB said the clips involved were installed after faults were found in earlier fasteners.

In the late 1980s, the Federal Aviation Administration ordered operators of 757s to install a redesigned fastening system. At the time, Eastern Airlines, which operated the 757 involved in the incident, installed the redesigned system, the NTSB said. It was these redesigned fasteners that failed on Flight 1250, the safety board said.

The safety board is continuing to evaluate the design, installation, inspection and maintenance of the failed components to determine the cause of the failure and the effect on the 757 fleet.E-mail to a friend